Can the HOA board meet without notifying homeowners?

No.  California law is explicit on this matter.  From time to time the Board may meet in an Executive Session, from which homeowners are excluded, to deal with a specific set of matters: primarily contract negotiations or matters pertaining to financial obligations of a particular homeowner (e.g., non-payment of HOA dues). California law restricts board members’ interactions outside of posted HOA meetings; in particular, a quorum of board members (at WCP a quorum is three) may not discuss HOA issues outside of an announced meeting.